Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for dampening a torsional vibration that appears in a rotating system.
The oil and gas industry has a growing demand for driving various machines at variable speeds. Such machines may include compressors, electrical motors, expanders, gas turbines, pumps, etc. Variable frequency electrical drives increase energy efficiency and provide an increased flexibility for the machines. For example, a mechanism for driving a large gas compression train is the load commutated inverter (LCI). A gas compression train includes, for example, a gas turbine, a motor, and a compressor. The gas compression train may include more or less electrical machines and turbo-machines. However, a problem introduced by power electronics driven systems is the generation of ripple components in the torque of the electrical machine due to electrical harmonics. The ripple component of the torque may interact with the mechanical system at torsional natural frequencies of the drive train, which is undesirable.
A torsional oscillation or vibration is an oscillatory angular motion that may appear in a shaft having various masses attached to it as shown for example in FIG. 1. FIG. 1 shows a system 10 including a gas turbine 12, a motor 14, a first compressor 16 and a second compressor 18. The shafts of these machines are either connected to each other or a single shaft 20 is shared by these machines. Because of the impellers and other masses distributed along shaft 20, a rotation of the shaft 20 may be affected by torsional oscillations produced by the rotation with different speeds of the masses (impellers for example) attached to the shaft.
As discussed above, the torsional vibrations are typically introduced by the power electronics that drive the electrical motor. FIG. 1, for example, shows a power grid source (power source) 22 providing electrical power to the LCI 24, which in turn drives the shaft 20 of the motor 14. The power grid may be an isolated power generator. in order to damp (minimize) the torsional vibrations, as shown in FIG. 2 (which corresponds to FIG. 1 of U.S. Pat. No. 7,173,399, assigned to the same assignee as this application, the entire disclosure of which is incorporated here by reference), an inverter controller 26 may be provided to an inverter 28 of the LCI 24 and may be configured to introduce an inverter delay angle change (Δβ) for modulating an amount of active power transferred from inverter 28 to motor 14. Alternatively, a rectifier controller 30 may be provided to a rectifier 32 and may be configured to introduce a rectifier delay angle change (Δα) for modulating the amount of active power transferred from the generator 22 to a DC-link 44 and thus to the motor 14. It is noted that by modulating the amount of active power transferred from the generator 22 to the motor 14 it is possible to damp the torsional vibrations that appear in the system including motor 14 and compressor 12. In this regard, it is noted that shafts of motor 14 and gas turbine 12 are connected to each other while a shaft of generator 22 is not connected to either the motor 14 or compressor 12.
The two controllers 26 and 30 receive as input, signals from sensors 36 and 38, respectively, and these signals are indicative of the torque experienced by the motor 14 and/or the generator 22. In other words, the inverter controller 26 processes the torque value sensed by sensor 36 for generating the inverter delay angle change (Δβ) while the rectifier controller 30 processes the torque value sensed by the sensor 38 for generating the rectifier delay angle change (Δα). The inverter controller 26 and the rectifier controller 30 are independent from each other and these controllers may be implemented together or alone in a given system. FIG. 2 shows that sensor 36 monitors a part (section) 40 of the shaft of the motor 14 and sensor 38 monitors a shaft 42 of the power generator 22. FIG. 2 also shows the DC link 44 between the rectifier 32 and the inverter 28.
However, it is possible to use other devices and methods for generating damping in the drive train.